Ball joints are found in automotive vehicles in two common applications, in steering linkages, between a tie rod and a steering arm, and in vehicle suspensions, between a suspension control arm and a steering knuckle. In either application, a ball joint generally includes a stud having a spherical head with a shank extending from the head, and a socket having a cavity within which the stud head is supported and turns. For purposes of illustration, the spherical stud head may be divided into two hemispheres, the hemisphere from which the shank extends being called the shank hemisphere, and the hemisphere opposite the shank being called the free hemisphere. It is known to assemble a ball joint by injection molding the socket directly around the spherical stud head. In such a case, a preformed socket cavity is not necessary because the socket material will automatically form its own cavity in the shape of the outer surface of the stud head during molding. With injection molding, the automatically formed cavity will inherently wrap around and provide a bearing seat to support as much of both stud head hemispheres as is desired. However, it is often preferable that the socket be preformed with a cavity into which the stud may be later inserted, so that the exact shape of the socket and socket cavity may be more closely controlled. In such a case, the stud has to be inserted through an opening into the cavity of the preformed socket in order to assemble the ball joint. The two possible methods of inserting the stud are head first or shank first. The United States patents disclose examples of each.
Examples of head first stud insertion may be seen in the U.S. Pat. Nos. 3,309,117 to Gottschald and 3,355,787 to Sullivan, also assigned to the assignee of the present invention. It will be understood that with head first insertion, only one opening into the socket cavity is strictly necessary, an opening large enough to admit the spherical head of the stud. However, Gottschald clearly illustrates a disadvantage with this method of assembly. The preformed socket may be easily molded with a cavity shaped to conform to and support the entire free hemisphere of the stud head. However, if the socket cavity is to be substantially rigid and still admit the stud head freely, it cannot be molded with a cavity designed to wrap around and support any substantial portion of the shank hemisphere of the stud head. A ball joint assembled with the head first method of insertion may, therefore, not be suitable if the stud is to be subjected to anything but a compression load, that is, anything but a load that forces the stud head into the socket cavity.
There are two known ways, with head first insertion, of providing a preformed socket with a cavity that also wraps and supports a substantial portion of the shank hemisphere of the stud head. One way is to mold the socket of a material that will yield significantly as the stud head is inserted, flexing past the largest diameter of the stud head, and then wrapping back around part of the shank hemisphere. Another way is to form part of the socket cavity around the shank hemisphere after the stud head has been inserted, as a subsequent assembly step. The U.S. Pat. No. 3,226,142 to Herbenar shows an example of a socket of yieldable material with an opening to the cavity that is smaller than the diameter of the stud head, so that the socket cavity will flex and wrap around part of the shank hemisphere as the socket is forced over the stud head. Of course, there would be limitations on the rigidity, strength and thickness of such a yieldable socket. In Sullivan, a subsequent cold forming step is carried out in order to form part of the socket material around the shank hemisphere of the stud head. Although this socket design provides good support to the stud head, it would be desirable to eliminate that assembly step, if possible.
Examples of the shank first method of inserting a stud into a preformed socket cavity may be seen in the U.S. Pat. Nos. 3,846,032 to Harada et al and 3,833,309 to Hobbs. With shank first insertion, there are two opposed openings into the socket cavity, a smaller one large enough to admit the stud shank, but not the stud head, and a larger one large enough to admit the stud head. The socket cavity generally includes a first bearing seat adjacent to the smaller opening and engageable with the shank hemisphere of the stud head. The ball joint is assembled by inserting the stud shank first through the larger opening until the shank hemisphere of the stud head engages the first bearing seat with the stud shank extending through the smaller cavity opening. One of the benefits of a ball joint assembled in this fashion is that support for the shank hemisphere of the stud head may be easily provided by the first bearing seat. The first bearing seat may be preformed and shaped as desired, and no subsequent shaping steps will be necessary. The engagement of the shank hemisphere with the first bearing seat also effectively closes the smaller socket cavity opening, which provides what is termed a travel window for the outwardly extending shank. However, there is the converse problem here of providing support to the free hemisphere of the stud head. Another potential drawback is the larger socket cavity opening, which it is necessary to cover, and which it is desirable to securely seal to retain lubricant around the stud head and keep contaminants out of the socket cavity. In Harada et al, the larger opening is covered with a lower housing member 4 that is joined to an upper housing member 3 around the outside of the socket 1. Also, a separate second bearing seat is provided for the free hemisphere of the stud head. However, the larger opening in the socket 1, although covered, is not directly sealed or plugged. In Hobbs, the larger opening is directly sealed by a plug 12, which is secured by sonic welding into the opening. However, there is nothing to back up the securement of the plug 12, which depends entirely on the strength of the sonic weld.